diff --git a/classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java b/classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java
index 1752ccd70..4fee5f51c 100644
--- a/classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java
+++ b/classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java
@@ -674,7 +674,7 @@ class TAbstractStringBuilder extends TObject implements TSerializable, TCharSequ
 
     public int indexOf(TString str, int fromIndex) {
         int sz = length - str.length();
-        outer: for (int i = fromIndex; i < sz; ++i) {
+        outer: for (int i = fromIndex; i <= sz; ++i) {
             for (int j = 0; j < str.length(); ++j) {
                 if (buffer[i + j] != str.charAt(j)) {
                     continue outer;
diff --git a/tests/src/test/java/org/teavm/classlib/java/lang/StringBuilderTest.java b/tests/src/test/java/org/teavm/classlib/java/lang/StringBuilderTest.java
index dc95c9f9f..aab08bacf 100644
--- a/tests/src/test/java/org/teavm/classlib/java/lang/StringBuilderTest.java
+++ b/tests/src/test/java/org/teavm/classlib/java/lang/StringBuilderTest.java
@@ -407,4 +407,14 @@ public class StringBuilderTest {
     public void substringWithUpperBoundAtEndWorks() {
         assertEquals("23", "123".substring(1, 3));
     }
+
+    @Test
+    public void indexOf() {
+        StringBuilder sb = new StringBuilder();
+        sb.append("12345");
+        assertEquals(3, sb.indexOf("45"));
+        assertEquals(-1, sb.indexOf("56"));
+        assertEquals(0, sb.indexOf("12345"));
+        assertEquals(0, sb.indexOf("123"));
+    }
 }